This is a working document prepared by NIOSH or its contractor for use in discussions with the ABRWH or its 

Working Groups or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or 

ABRWH (or their technical support and review contractors) positions unless specifically marked as such. This 

document represents preliminary positions taken on technical issues prepared by NIOSH or its contractor. 

NOTICE: This report has been reviewed to identify and redact any information that is protected by the 

Privacy Act 5 USC §552a and has been cleared for distribution.

1

WHITE PAPER

TBD 6000 Review by SC&A 

Determination of Settling Time

Prepared by David Allen, DCAS

August 2013

BACKGROUND

Battelle-TBD-6000 includes a method for estimating surface contamination using measured airborne 

concentrations of uranium. As part of an earlier review of TBD-6000 by SC&A, the method prescribed by 

NIOSH was questioned and it was pointed out by SC&A that a good source of information to verify the 

method was contained in Adley (SRDB 27354). An analysis of Adley was performed by NIOSH and 

documented in the Issue 5 white paper dated October 9, 2009. The analysis concluded that a 30 day 

deposition time should be used with the technique. TBD-6000 was revised to include this settling period

and the issue raised by SC&A was closed by the TBD 6000 work group.

On May 15, 2013 a supplemental review was issued by SC&A that reanalyzed the technical bases behind 

TBD 6000. In this review SC&A concluded that the 30 days period may not be sufficient to reach 

equilibrium between the deposition and removal processes. This white paper explores this new 

analysis.

SC&A REVIEW

Using Stoke’s Law, the new SC&A review derived a settling rate of 0.00052 m/s and concluded that the 

0.00075 m/s settling rate used in TBD-6000 was claimant favorable. The settling time to reach 

equilibrium was then determined, at each of 17 plant locations, using the 0.00052 m/s settling rate and 

an assumed uniform, plant-wide airborne concentration of 1400 μg/m3

. The analysis resulted in a 

variety of settling times at each location, ranging from 13 days to 73 in Table 4 of the report. 

NIOSH does not believe the parameters used in this analysis are appropriate for this application. As 

SC&A points out, the equilibrium surface contamination is the product of the airborne activity, the 

settling velocity and the settling time. Since TBD-6000 provides both the settling velocity and the 

settling time, the product of these two values is really the key parameter rather than the individual 

values themselves. If TBD 6000 used the 0.00052 m/s settling rate, it is agreed that a longer settling 

time would be required to reach equilibrium. Because TBD 6000 uses a faster settling velocity, the time 
This is a working document prepared by NIOSH or its contractor for use in discussions with the ABRWH or its 

Working Groups or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or 

ABRWH (or their technical support and review contractors) positions unless specifically marked as such. This 

document represents preliminary positions taken on technical issues prepared by NIOSH or its contractor. 

NOTICE: This report has been reviewed to identify and redact any information that is protected by the 

Privacy Act 5 USC §552a and has been cleared for distribution.

2

to reach equilibrium will be fewer days. The product of the two parameters, however, remains the 

same.

It should also be noted that the airborne level used in the SC&A review comes from a quote in Adley 

that is apparently attempting to describe the significance of surface contamination. It is not actually a

measurement of the airborne activity at the facility. The report estimated 20 grams per day of uranium 

surface contamination settling throughout the building and pointed out that if all of this were evenly 

distributed throughout the building volume, the airborne concentration would be 28 times the 

maximum permissible level (1400 μg/m3

). The exact quote from the report is:

“The significance of the quantities of uranium oxide dust that settle out is realized when the following 

two examples are considered:

(1) If the 20 grams of U3O8 that settles out daily were dispersed evenly throughout the 500,000 

cubic feet of the building, the resulting concentration of dust would be 28 times the 

maximum permissible level of 5 x 10-5 μg/cc.

(2) When one milligram of U3O8 is dispersed in the air, the quantity is sufficient to produce the 

maximum permissible concentration in 710 cubic feet of air; in one cubic foot of air it will 

produce 710 times the maximum level.”

As such, the statement is an example to demonstrate the potential significance of the quantity of

surface dust. With the second example included, it is clear this is not an estimate of airborne activity 

but rather examples of the possible significance ranging from 1 MPC to 710 MPCs.

The third issue with the SC&A analysis is that the recommendation does not account for a distribution of 

air samples. When air sample values are available for a site, one of two standard approaches is use. 

Either the 95th percentile is used as a constant or the full distribution of the air samples is used. The 

SC&A report recommends using 70 days for the settling time based on the 95th percentile of the settling 

times calculated in the report. 

An example of the problem comes from applying the data from Adley as it would typically be applied by 

NIOSH at a facility with no measured contamination value. The 95th percentile of the airborne values 

measured by Adley is 26,757 μg/m3 (as determined later in this paper). If a contamination value were to 

be determined from this using the SC&A recommendations, this value would be multiplied by 70 days 

and 0.00075 m/s settling rate to obtain a contamination value of 1.21x108 μg/m2

. This is more than 26 
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Working Groups or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or 

ABRWH (or their technical support and review contractors) positions unless specifically marked as such. This 

document represents preliminary positions taken on technical issues prepared by NIOSH or its contractor. 

NOTICE: This report has been reviewed to identify and redact any information that is protected by the 

Privacy Act 5 USC §552a and has been cleared for distribution.

3

times higher than the maximum surface concentration reported by Adley (as shown in Table 2 of the 

SC&A report).

Since the SC&A recommendation was based on an airborne value that was hypothetical rather than 

measured, and a settling rate not used by TBD-6000, it does not appear to be a reliable estimate of the 

appropriate setting time for NIOSH to use. Also, since it does not appear to take into account the 

distribution of the air samples at a site, it may not be practical to use. Therefore, as recommended in 

the SC&A report, the settling time is re-examined in the remainder of this paper.

ISSUE 5 WHITE PAPER

The issue 5 white paper explored the appropriate settling rate and settling time based on contamination 

data from Adley and Simonds Saw and Steel. In that paper, settling time based on both the geometric 

mean (GM) and the 95th percentile values were calculated and the most favorable of the values was 

used. However, since the analysis was performed as part of the review of TBD-6000, airborne values 

contained in TBD-6000 were used in the analysis. This creates the possibility of the technique being 

used at a site with site specific airborne data but TBD-6000 parameter values (settling time and settling 

rate). Therefore, the analysis is expanded here to consider site specific airborne values from Adley and 

Simonds Saw and Steel.

ANALYSIS

After conversion to units of μg/m3

, the air sample values contained in Tables II through VII of the 

appendix to Adley were used in this analysis. Samples from Table VIII were not included because they 

contained samples from non-production areas (e.g., office and restroom). In addition, the 

contamination values from the settling plate data (Table XIII) were used. The settling data, listed in 

mg/ft2

/day, was multiplied by the applicable number of days the plate was exposed and converted to 

μg/m2

. Simonds Saw and Steel air sample data was obtained from an October 27, 1948 survey (SRDB 

10883 pp. 19-25), while the contamination values were obtained from a summary of three Simonds Saw 

and Steel surveys (SRDB 23579 Table II). As with the Adley data, the air sample results were converted 

to μg/m3 and the contamination values were converted to μg/m2

.

Each of these four data sets was analyzed as a lognormal distribution and the geometric mean (GM) and 

the geometric standard deviation (GSD) were determined. The settling time was then determined using 

the following equation.
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Working Groups or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or 

ABRWH (or their technical support and review contractors) positions unless specifically marked as such. This 

document represents preliminary positions taken on technical issues prepared by NIOSH or its contractor. 

NOTICE: This report has been reviewed to identify and redact any information that is protected by the 

Privacy Act 5 USC §552a and has been cleared for distribution.

4

Settling time = Surface contamination ÷ airborne value ÷ 0.00075 m/s 

In determining the settling time, the values used for the surface contamination and airborne levels were 

from the same site (Adley or Simonds Saw and Steel) and same point in the distribution. For example, 

the GM of the Adley airborne and the GM for the Adley contamination data were used in the equation. 

Settling time was determined for the GM as well as the 95th percentile of the distributions for each site. 

Also, as discussed later, settling time was determined for the average values from the data sets. Table 1 

provides the calculated settling times from the Simonds saw and Steel and the Adley data. 

Table 1 – Settling Days

Data 

Source

Distribution 

parameter

Surface Contamination 

(μg/m2)

Airborne Concentration 

(μg/m3) Settling Days

Simonds 

Saw and 

Steel

GM 1005276 841 18.45

Average 1552083 5204 4.60

95th 5324129 27675 2.97

Adley GM 1839873 534 53.13

Average 2350865 8436 4.30

95th 5645961 26757 3.26

As can be seen in Table 1, the settling time for the Adley data is 53 days when the GM of the 

distributions are used and 3.26 days when the 95th percentile of the distributions are used. The disparity 

in the values is due to the large difference in the GSDs of the two distributions, which directly affects the 

calculated 95th percentile values. It is likely that air samples tend to have a wider distribution due to 

localized sources of airborne. Once released however, that airborne would tend to mix and migrate,

causing the settled contamination to be more evenly distributed.

The GM of the surface contamination would not normally be used alone for exposure estimates. 

Instead, either a full distribution or a constant at the 95th percentile value would be used. Therefore, 

the GM is not necessarily a good parameter to use in determining the appropriate settling time.
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Working Groups or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or 

ABRWH (or their technical support and review contractors) positions unless specifically marked as such. This 

document represents preliminary positions taken on technical issues prepared by NIOSH or its contractor. 

NOTICE: This report has been reviewed to identify and redact any information that is protected by the 

Privacy Act 5 USC §552a and has been cleared for distribution.

5

It is clear from the table that neither the GM nor the 95th percentile will describe the appropriate 

settling time for a full distribution of airborne and contamination values. In an attempt to relate the full 

distribution to single value, the use of the average of the distribution was evaluated. The average of 

each distribution was calculated from the GM and GSD of each distribution using the equation from 

Table 2.2 in Battelle-TIB-5000 and reproduced in the current SC&A review. Based on the average of the 

air samples and the average of the contamination values, the number of settling days using the Adley 

data would be 4.3 days. 

Another set of data examined in the Issue 5 white paper was the Simonds Saw and Steel data. The 

values calculated for this data are also provided in Table 1. The number of settling days determined 

from this data is less than 30 days for each of the three parameters in the table.

Attachment A to this white paper provides an evaluation of a hypothetical case to determine what 

constant value would be equivalent (i.e., result in the same PC value) to a lognormal distribution when 

entered into IREP. The examination determined that the constant value would fall somewhere between 

the average and the 95th percentile of that distribution. This implies that the appropriate settling time 

based on Adley and Simonds Saw and Steel data falls somewhere between 2.97 days and 4.6 days. 

The issue 5 white paper used the GM and the 95th percentile values and resulted in a 27 day estimate 

(rounded to 30 days) based on the GM. However, as appendix A demonstrates, the average would be a 

more appropriate value and would result in a settling time of 7.4 days rather than 27 days. 

CONCLUSION

If the 95th percentile were to be used, both the Adley and the Simonds Saw and Steel data indicate a 30 

day settling time is favorable. If the full distribution is to be used, the Simonds Saw and Steel data 

indicates a 30 day settling time is favorable while the Adley data is unclear. It is unclear because the GM 

indicates it is not favorable while the 95th percentile indicates it is favorable.

The data therefore appears to give mixed results. This is true, however, only if the GM is to be used as 

a constant which is not normally the case. Based on Appendix A, the constant value that is equivalent to 

a lognormal distribution (for the purposes of IREP) would fall somewhere between the average of the 

lognormal distribution and the 95th percentile of that distribution. Using these parameters, the 30 day

settling time is favorable for both the Adley data and the Simonds Saw and Steel data.
This is a working document prepared by NIOSH or its contractor for use in discussions with the ABRWH or its 

Working Groups or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or 

ABRWH (or their technical support and review contractors) positions unless specifically marked as such. This 

document represents preliminary positions taken on technical issues prepared by NIOSH or its contractor. 

NOTICE: This report has been reviewed to identify and redact any information that is protected by the 

Privacy Act 5 USC §552a and has been cleared for distribution.

6

Based on this limited reanalysis of the data, it is recommend that the 30 days settling time be retained in 

TBD-6000. It is recommended that the value not be lowered at this time until further analysis of 

possible additional datasets can be completed at some date in the future. 
This is a working document prepared by NIOSH or its contractor for use in discussions with the ABRWH or its 

Working Groups or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or 

ABRWH (or their technical support and review contractors) positions unless specifically marked as such. This 

document represents preliminary positions taken on technical issues prepared by NIOSH or its contractor. 

NOTICE: This report has been reviewed to identify and redact any information that is protected by the 

Privacy Act 5 USC §552a and has been cleared for distribution.

7

ATTACHMENT A

The following analysis was undertaken to determine if a single constant value could be found that 

approximates the same result at the 99th percentile as the input of a lognormal distribution into IREP. 

The analysis utilized a hypothetical case entered into IREP. The details of the hypothetical case are:

Born - 1930

Exposure – 10 years from 1961 to 1970

Date of cancer diagnosis – 2000

Type of cancer – lung

Other details – non-smoker, alpha radiation, chronic exposure

Dose was entered into IREP as a lognormal distribution with a GM of 1 rem and a GSD that varies as 

shown in the table below. The Probability of Causation (POC) was determined for each of several 

distribution that varied only by GSD. The annual dose was identical for each of the 10 years for each 

IREP run. The GM was 1 rem for each year for each IREP run but the average and 95th percentile doses 

were different for each run due to a different GSD being used. These are included in the table below. 

For each distribution, the POC was determined and recorded in the table. Next, the distribution type 

was changed from lognormal to constant and a value was determined for each distribution that would 

produce the same POC as the lognormal distribution. Each of these doses was entered into IREP and the 

POC recorded in the table to verify the POC did indeed result in in the same POC as the lognormal 

distribution. 

It can be seen from the data in the table that for a wide range of GSDs, the constant dose necessary to 

produce the same POC always falls between the average and the 95th percentile of the distribution.

Lognormal Distribution Constant Value

Rem/year GSD POC (%) Average 95th Rem/year POC (%)

1 2 41.91 1.272 3.127 1.403 41.91

1 4 63.17 2.614 9.781 3.335 63.17

1 6 78.63 4.979 19.057 7.155 78.63

1 8 87.03 8.689 30.590 13.049 87.03

1 10 92.19 14.167 44.157 22.955 92.19